In these years, the optical transmission technique is widely used not only in major transmission networks but also in LANs (Local Area Networks), home networks, or access systems. For example, in Ethernet, a transmission rate of 10 Gbps has been obtained, a higher transmission rate will be required in the near future, and an optical transmission system is expected to provide a transmission capacity higher than 10 Gbps.
In a light source used in optical transmission having a transmission rate lower than 10 Gbps, a direct modulation scheme is primarily employed in which the power of output light is modulated by modulating an injection current flowing in a semiconductor laser. However, it is difficult to operate a semiconductor laser at a modulation frequency higher than 10 GHz with the direct modulation scheme.
To solve this problem, a modulation scheme has been developed in which light output from a semiconductor laser is modulated by an external modulator. However, in this external modulation scheme, size of modules is large, and a large number of parts are used; hence, cost is high. For this reason, even though an optical transmission technique using an external modulator is applied in expensive systems such as major transmission networks, it is not suitable for personal systems, such as LANs or home networks.
On the other hand, it has been studied to use a vertical cavity surface emitting laser (VCSEL) as a light source used in LANs or for optical interconnections. The VCSEL has low power consumption compared to traditional edge-emitting lasers, and because cleavage is not needed during the fabrication process, it is possible to inspect the device in wafer form, hence the fabrication cost can be reduced. For this reason, it is expected to use a vertical cavity surface emitting laser (VCSEL) by means of direct modulation as a light source in LANs or for optical interconnections involving a transmission capacity higher than 10 Gbps.
The following methods have been proposed to perform fast speed modulation for the VCSEL.
Japanese Laid-Open Patent Application No. 2002-185079 (referred to as “reference 1” hereinafter) discloses a technique in which a current is injected into an active region without passing through an upper multilayer reflecting mirror, and a modulation doped stacked structure able to generate two-dimensional carriers is provided so as to reduce a lateral resistance, and this reduces the resistance of the VCSEL and spreads an electrical modulation region, which is restricted by a resistance R and a capacitance C.
Japanese Laid-Open Patent Application No. 2002-204039 (referred to as “reference 2” hereinafter) discloses a technique in which a quantum well layer capable of intersubband absorption is arranged near a light emission layer; when modulated optical signals are input, due to the intersubband absorption of the quantum well layer, a carrier distribution, a carrier density of the light emission layer, and light emission output are modulated; as a result, high speed is obtainable with the response speed not being influenced by a CR time constant or a carrier transportation effect.
Japanese Laid-Open Patent Application No. 7-249824 (referred-to as “reference 3” hereinafter) discloses a technique in which horizontal cavity semiconductor lasers for optically exciting a VCSEL are stacked on the same substrate, a forbidden band width of an active layer of the VCSEL is set to be less than a forbidden band width of the horizontal cavity semiconductor lasers to increase optical excitation efficiency, and modulation optical signals from the horizontal cavity semiconductor lasers are input for external modulation; thereby, the modulation frequency of the VCSEL is increased.
In IEICE technical report, Optoelectronics 2003-218, LQE3003-155 (referred to as “reference 4” hereinafter), a technique is disclosed in which a DFB laser beam is injected into a VCSEL and is synchronized with an oscillation wavelength of the VCSEL (this is the so-called “injection lock”); due to this, a relaxation oscillation frequency of the VCSEL is increased to be 22.8 GHz.
As described above, in reference 1, response speed is improved when electrical modulation signals output from a driver circuit modulate the carrier density of the light emission layer of the VCSEL. In references 2 and 3, the optical modulation signals are input from outside to reduce influence of a delay of an electrical response speed when a current flows into the device.
However, none of the techniques in references 1, 2 and 3 can increase the relaxation oscillation frequency, which represents a frequency at which the speed of induced emission can no longer follow the change of the carrier density in the light emission layer. Due to this, the modulation speed is limited by the relaxation oscillation frequency.
In reference 4, a laser beam is injected from outside and is synchronized with the oscillation mode of the VCSEL; thereby, the photon density inside the VCSEL is increased. Since the internal photon density is associated with the relaxation oscillation frequency, by increasing the internal photon density, the relaxation oscillation frequency of the VCSEL can-be increased. However, in the technique shown in reference 4, it is necessary to accurately set the wavelength of the laser beam injected from outside to be exactly equal to the wavelength of the oscillation mode of the VCSEL. Even when the wavelength deviates by only a few nanometers to be out of synchronization, it is impossible to increase the relaxation oscillation frequency of the VCSEL. For this reason, devices for tuning the wavelength and for temperature control are required, thus the overall device become complicated.